1. Field of the Invention
The present invention relates to an image forming method and apparatus for generating a recording fluid having a predetermined density and/or a predetermined color by changing a proportion or mixing ratio of plural coating liquids based on an image signal, and leading the thus-obtained recording fluid to an image receiving medium to form an image.
2. Description of the Related Art
U.S. Pat. No. 3,416,153 (which will be referred to as a prior art reference 1, hereinafter) discloses an image forming method, in which a series of charged ink droplets having predetermined intervals is caused to pass through an electric field modulated by an image signal. Unnecessary ink droplets are deflected to be removed and desired ink droplets are selectively guided to a recording sheet so as to form an image on the recording sheet. Since the ink droplets are continuously ejected or jetted in this system, this is referred to as a continuous ink jet system.
U.S. Pat. No. 3,946,398 (which will be referred to as a prior art reference 2, hereinafter) discloses a recording method, in which a piezoelectric transducer plate is deformed by a modulation of an image signal to push out the ink. The pushed-out ink droplets are jetted or expelled from an orifice to be impacted on a recording medium. This system is referred to as a piezo ink jet system.
U.S. Pat. No. 4,490,728 (which will be referred to as a prior art reference 3, hereinafter) discloses another recording method, in which the ink is rapidly expanded or vaporized by heat of a heater modulated by an image signal. The rapidly-expanded ink gas or vapor is used to jet the ink liquid from an orifice to be impacted on a recording medium. Since ink droplets are jetted by using heat, this is referred to as a thermal ink jet system.
U.S. Pat. No. 4,109,282 (which will be referred to as a prior art reference 4) discloses a printing device, in which a valve called a flap valve is provided in a flow path for leading two types of liquid, i.e., clear ink and black ink into a substrate for forming an image. The flow path for each ink is opened/closed by displacement of this valve so that the two types of liquid are mixed in a desired density to be transferred onto the substrate. This enables printout of an image having the gray scale information which is the same with that of the image information displayed on a TV screen.
This reference 4 discloses that a voltage is applied between the flap valve and an electrode provided on a surface opposed to the flap valve and the valve itself is mechanically deformed by the electrostatic attracting force to cause displacement of the valve. Further, the ink is absorbed in paper by a capillary action which acts on the ink between a tip of the flap valve and fibers of the print paper.
Unexamined Japanese Patent Publication (KOKAI) No. 291663/1988 (which will be referred to as a prior art reference 5, hereinafter) discloses a coating method, in which two types of thick (dark) and thin (light) liquid are mixed in a coating head to be continuously extruded from a slot-opening opposed to a running web. Thus, the mixed liquid is consecutively coated on the web. In this coating method, the mixed liquid is coated over the entire coating width with a uniform coating membrane pressure without forming a residue deposit, and the coating liquid having a density graduation in time course is continuously applied with respect to a traveling direction of the web. In addition, this method enables coating with a uniform thickness with respect to the width direction.
Accordingly to the method disclosed in the prior art reference 1 (the continuous ink jet system), unnecessary ink droplets are removed by modulating the electric field to enable drawing of a desired image. However, it is required to provide each mechanism for independently modulating the electric field for each nozzle provided for each pixel, thereby making it difficult to reduce the dimension of each nozzle. It is also hard to form multiple nozzles with a high density in accordance with pixels. Only a part of continuously jetted ink droplets must be used for forming an image, and hence this mode is not suitable for high speed recording because many ink droplets are not used but removed. Moreover, since the ink is continuously jetted, a large amount of ink is wasted, and the obtained is thus expensive.
According to the method disclosed in the prior art reference 2 (the piezo ink jet mode), a desired image can be drawn by jetting only ink droplets which are used for forming an image. Jetting only a necessary amount of ink eliminates the waste of ink, and a relatively-inexpensive print can be obtained. However, the nozzles must be arranged in the high density for realizing the high quality of an image, leading to a problem where the image is distorted by the interaction of the ink droplets jetted from adjacent nozzles.
According to the method disclosed in the prior art reference 3 (the thermal ink jet mode), an arbitrary image can be drawn and jetting only a necessary amount of ink can obtain a relatively-inexpensive print as similar to the above-mentioned piezo ink jet mode. However, when the nozzles are provided in the high density for realizing the high quality of an image, the image is distorted by the interaction of the jetted ink droplets. Additionally, in the above prior art references 1-3, since the droplets are jetted onto image receiving paper at high speed, a part of the ink droplets smashes by impact to form an ink mist. Such ink mist cannot be captured on the image receiving paper. The uncaptured ink mist leaks to the installation environment of the printer to pollute the environment which is pointed out as a problem.
According to the method disclosed in the prior art reference 4, the ink extruded from the nozzle is directly applied on the paper. Therefore, in a case where the paper has a large thickness or irregularity on the surface of the paper, it is difficult to reproduce an image on the paper with fidelity with respect to the electric signal. Accordingly, this method is not done in practical use, as yet. Further, since the ink to be used is restricted to two types, a color image cannot be recorded. Furthermore, since the ink is drawn out by the capillary action between the ink and the fibers of the paper in this mode, the ink tends to be affected by the quality of the paper and a change in quality of the paper involves a change in quality of an image. Moreover, the image cannot be truly reproduced due to the partial irregularity of the fiber structure even if the paper with the same quality is used.
According to the coating method disclosed in the prior art reference 5, although an image having a density graduation along a traveling direction of a web which is a target of coating can be formed, the image cannot have a density graduation along a width direction of the web (a direction orthogonal to the web traveling direction). Consequently, application of the coating liquid whose color or density changes for each pixel in accordance with an image signal is impossible.
The present invention has been accomplished under the circumstances as aforementioned, and a first object thereof is to provide an image forming method by which: a high-quality image can be formed at high speed; a coating liquid cannot be wastefully used; reduction in dimension of a nozzle is possible; an installation environment cannot be adversely affected; influence of a thickness, a state of a surface or an undulatory surface of a final image receiving medium such as paper or irregularity of the fiber structure of the same can be eliminated; and an image can be stably formed.
Further, it is a second object of the present invention to provide an image forming apparatus which is directly used for implementing this method.
According to the present invention, the first object can be attained by an image forming method for forming an image on an image receiving medium with plural coating liquids, comprising the steps of;
a) providing an array of plural extruding ports aligned in a direction substantially orthogonal to a relative movement direction of the image receiving medium;
b) combining said plural coating liquids extruded in each of the plural extruding ports to form a recording liquid and extruding said recording liquid from each of said plural extruding ports, a mixing ratio of said plural coating liquids in the recording liquid being varied based on an image signal; and
c) transferring said recording liquid to said image receiving medium as a continuous flow while said image receiving medium is moved relatively to said aligned plural extruding ports;
whereby said recording liquid constituted by the plural coating liquids is continuously applied on said image receiving medium to form the image.
Plural coating liquids may be mixed homogeneously to form the recording liquid to be coated on the image receiving medium, such as a recording sheet or a temporary (intermediate) image receiving medium. Preferably, however, the recording liquid is not homogeneous mixture of the plural coating liquids. Rather, the recording liquid has layer construction of a laminar flow of the plural coating liquids. Specifically, the plural coating liquids extruded from the respective extruding ports may be continuously applied to the image receiving medium in a direction of a thickness of coating in the superimposed manner without homogeneously mixing the coating liquids.
The extruding ports may be provided in accordance with respective pixel to be aligned in a direction of the width of an image receiving medium (a direction substantially orthogonal to a relative-displacement direction). Thus aligned plural extruding ports may be formed in a slot-shaped opening, and the coating liquids extruded from each extruding ports associated with each pixel are integrated and zonated in the slot-shaped opening along the width direction. The zonated composite liquid of coating liquids can be thereby applied on the image receiving medium.
At least one of the plural coating liquids may be clear liquid which is substantially transparent or becomes substantially transparent when dried out. A density of pixels in the coated image can be controlled by a proportion or mixing ratio of this clear liquid and non-clear coating liquid. The non-clear coating liquid is a liquid different from the clear liquid and has an optical density. In this case, by maintaining a volume flow rate of the coating liquids to a substantially-fixed value, the flow of the coating liquids can be smoothed to prevent the image quality from being reduced. Furthermore, at least one clear liquid may be fed at a substantially constant feed pressure irrespective of an image signal. The extrusion amount of this clear liquid can be controlled to be changed by controlling an extrusion amount of the other coating liquid(s) to be mixed or combined with this clear liquid. As properties of multiple types of the coating liquid to be used, it is preferable that these types of the coating liquid are superimposed or laminated to be applied in a direction of the coating thickness, and those having small differences in characteristics at least in viscosity, specific gravity, surface tension and temperature are desirable. The superimposed state cited above includes the state in which the adjacent two types of the coating liquid are mixed with a range of a minute distance from a border.
In multiple types of the coating liquid extruded in the superimposed or laminated manner, the coating liquid in at least one outermost layer may be a clear liquid which is or becomes substantially transparent when dried out. With such an arrangement, the influence of irregularity of the surface state and the like of the image receiving medium can be eliminated to improve the image quality by using this clear liquid as undercoating liquid coming into contact with the surface of the image receiving medium.
The image receiving medium may be an intermediate image receiving medium, such as a transfer drum, holding the composite layers of the coating liquids temporarily and then transferring the composite layers to a final image receiving medium such as recording sheet. In this case, the undercoating liquid in the outermost layer of the composite layers comes into contact with the surface of the intermediate image receiving medium, and then comes into contact with the top surface of the final image when transferred to the final image receiving medium.
On the contrary, the undercoating liquid may be superimposed so as to be the uppermost layer when applied on the intermediate image receiving medium. When transferred to the final image receiving medium, the undercoating liquid is brought into contact with the surface of the image receiving medium and undercoats the surface. Further, when coating by using the intermediate image receiving medium, arrangements are made so that the coating liquids can smoothly transfer to the final image receiving medium when the temporary formed image on the intermediate image receiving medium is transferred to the final image receiving medium. For example, adhesion between the intermediate image receiving medium and the coating liquid establishing contact therewith or cohesion in this coating liquid is so set as to be smaller than cohesion in or between other types of coating liquid or adhesion between the final image receiving medium and any other coating liquid establishing contact therewith.
When adjacent extruding ports are biased each other in a direction which is not orthogonal to the relative displacement direction of the image receiving medium, a distance between adjacent pixels can be narrowed to improve the image quality. In this case, the distortion or deviation of pixels in recorded image can be compensated by changing the clock timing of the image signal in accordance with an amount of bias of the adjacent extruding ports.
The flow of the coating liquids can be stabilized by always extruding the coating liquids from the respective extruding ports during a period in which no image is formed or recorded. The coating liquid which is unnecessary for formation of an image is removed and collected during the transfer from the respective extruding ports to the image receiving medium.
According to the present invention, the second object can be attained by an image forming apparatus for forming an image on an image receiving medium with plural coating liquids, comprising:
a recording head having an array of plural extruding ports aligned in a direction substantially orthogonal to a relative movement direction of the image receiving medium, the respective extruding ports extruding the plural coating liquids and combining the plural coating liquids extruded to form a recording liquid, the recording liquid being transferred to the image receiving medium as a continuous flow while the image receiving medium is moved relatively to said aligned plural extruding ports;
extrusion amount controlling means for controlling an amount of supply of said plural coating liquids fed to said respective extruding ports; and
a controller for controlling a mixing ratio of said plural coating liquids in the recording liquid based on an image signal and determining a supply amount and supply timing of the respective coating liquids, the determined supply amount and supply timing being fed to said extrusion amount controlling means;
whereby said recording liquid having the mixing ratio of the plural coating liquids based on the image signal is continuously applied on the image receiving medium to form the image.
The extrusion amount controlling means may be formed by an extrusion amount control valve provided in a passage extending from a feed path for supplying the coating liquid to the respective extruding ports. For example, it may be a diaphragm valve using a piezoelectric device. This extrusion amount control valve is provided for each pixel aligned in a direction of the width of the recording head and controls a quantity of flow by any of or combination of an opening, an opening time and a number of times of opening. Further, the extrusion amount controlling means may be formed by a pump whose quantity of extrusion is variable. This pump can be constituted by, for example, a piezoelectric device provided for each pixel aligned in a direction of the width of the recording head and a one-way valve. In this case, a quantity of flow is controlled by any of or combination of an operating speed, an operating time and a number of times of operation of the pump.
The plurality of coating liquids, needless to say, may be all separately controlled by an extrusion control valve comprising a control valve or a pump, but part of the coating liquid which is always extruded, for example, the clear liquid may be fed at a substantially constant pressure irrespective of an image signal. In this case, the feeding amount of the clear liquid which is fed at the substantially constant pressure is decreased or increased in accordance with the increase or decrease of the extrusion amount of the other coating liquid. That is to say, the total flow rate of all the coating liquids substantially depends on a diameter of a coating liquid extruding port, and hence, the extrusion amount of the clear liquid can automatically be controlled by the extrusion amount of the other coating liquid. In consequence, the number of the extrusion amount control means which are disposed on a recording head can be reduced, so that the constitution of the recording head can be simplified.
The plural extruding ports may be provided in accordance with each of the pixels aligned in a direction of the width of the image receiving medium.
The plural extruding ports may be divided into groups so that the respective groups corresponds to the respective pixels. Specifically, one group of the extruding ports is provided in a moving direction of the image receiving medium for one pixel so that multiple types of coating liquid having different colors or properties can be supplied from the respective extruding ports of the group. Also, the extruding ports provided for the multiple pixels may be divided into groups in a direction of the width of the image receiving medium in such a manner that extrusion of the recording liquid from a part of the groups is stopped in accordance with the width of the image receiving medium or the width of an image. In such a case, the wasteful consumption of the coating liquid can be prevented and, when the unnecessary coating liquid having no contribution to the coating process is removed and collected, an amount of liquid to be collected can be reduced.
The recording liquid, i.e., combined coating liquids can be transferred from the recording head to the image receiving medium by various kinds of modes. For example, it is possible to adopt a slot coating method by which the coating liquid extruding ports is formed on the top surface, the bottom surface or the side surface of the recording head and the image receiving medium is moved along the surface having the extruding ports with maintaining a predetermined gap from the surface. The coating liquids are extruded and guided to the gap between the surface of the recording head and the image receiving medium to form an image.
Additionally, it is possible to use a slide coating method by which a sloped surface which inclines toward the image receiving medium is formed on the top surface of the recording head and the coating liquids extruded on the sloped surface flow down to form a bead at the lower end of the sloped surface where the coating liquids meet the image receiving medium which is moving thereby, so that an image is formed or recorded on the image receiving medium. Moreover, a curtain coating method may be adopted, in which the coating liquids supplied from the recording head flow down along a guide plate onto the image receiving medium.
Although the image receiving medium itself may be a final image receiving medium such as print paper, it may be an intermediate image receiving medium. In this case, the intermediate image receiving medium is provided between the recording head and the final image receiving medium and transfer the coating liquids fed from the recording head to the final image receiving medium, and it may have a drum-like shape or an endless belt-like shape.
The controller determines a proportion of a mixture or a quantitative ratio of the coating liquids led to each extruding port based on an image signal and controls a color or a density of the mixed or combined liquid. A plurality of types of coating liquid are mixed or combined to form the mixed liquid (the recording liquid), which is extruded as a continuous flow from the extruding port and transferred to the image receiving medium. As a result, an image is formed on the image receiving medium. Since this recording liquid is applied as a continuous flow, the recording or coating liquid is not wasted and a high-quality image can be formed at high speed.
In the present invention, the image formed on the image receiving medium includes graphical intelligence patterns such as alphanumeric characters, graphical display, line art, and other image information.